The present invention relates to improvements in fluid power load-clamping systems for variably regulating maximum load gripping forces in a manner automatically adaptive to at least one characteristic of the load.
Various types of such adaptive load-clamping systems have been proposed in the past. Such previous systems can be categorized as follows:                (1) Systems which sense the existence of load slippage and respond automatically by gradually increasing the gripping force on the load by fixed force increments until the sensed slippage stops;        (2) Systems which automatically vary the gripping force in proportion either to the sensed weight or to the resistance to gripping of the load, without regard to whether or not slippage is actually occurring; and        (3) Systems which perform a combination of (1) and (2).        
Fluid power clamping systems of any of the above types regulate gripping force by gradually increasing gripping fluid pressure automatically from a relatively low threshold pressure. However such low threshold pressure limits the speed with which the load-engaging surfaces can be closed into initial contact with the load, thereby limiting the productivity of the load-clamping system. This problem occurs because high-speed closure requires higher closing pressures than the desired low threshold pressure, such higher pressures becoming trapped in the system by fluid input check valves during initial closure so that the desired lower threshold pressure is exceeded before automatic regulation of gripping pressure can begin. Although gripping pressure relief valve systems have in the past provided high and low relief settings selectable either manually, or automatically in response to clamp closure speed, to enable high-speed closure followed by low maximum gripping pressure, no such systems capable of automatically changing such settings in a manner compatible with automatic variable gripping pressure regulation have been known.
Prior fluid power systems such as those disclosed in British Patent Publication No. 2312417 and German Patent Publication No. 3245715, which vary the gripping fluid pressure in proportion to the sensed weight of the load, obtain weight measurements by lifting the load. However such weight-sensing systems operate only in response to clamp closure actuation, and therefore do not continue to vary the gripping fluid pressure in proportion to load weight during subsequent manipulation of the load in the absence of continued clamp closure actuation. Furthermore, such prior systems do not weigh the load in response to lifting of the load by tilting which, in paper roll handling operations, is a commonly-used alternative way to lift the load. The system shown in the British publication is also susceptible to inaccurate weight measurements due to variations in lifting pressure which are inherent within the extensible lifting mechanism depending upon its degree of extension.
Such prior weight-responsive systems also do not provide for different selectable predetermined relationships between the weight of the load and the gripping pressure, which are needed to account for variations in load fragility and stability.
Although automatic load tilt adjustment systems have been provided in the past for leveling fragile loads to prevent edge damage when the load is being set down, such automatic adjustment systems have not been capable of sensing the tilt of the load with respect to gravity,leading to inaccurate automatic tilt adjustment depending on whether or not an industrial lift truck is level with respect to its supporting surface, or whether or not such surface is level.
Valves for automatically preventing excessive lowering of the lifting mechanism when a clamped load is set down, to prevent subsequent damage to fragile load surfaces by downward slippage of the clamp when it is opened to disengage the load, have been provided in the past as shown, for example, in U.S. Pat. No. 3,438,308. However, such previous systems lack the versatility needed for reliable protection of the load under variable circumstances, such as variations in the degree of extension of the lifting mechanism when the load is set down.